Speed control device for AC electric motor

ABSTRACT

A speed control apparatus of an AC motor a first subtracter  1  for finding toque component voltage saturation amount ΔV q  from torque component voltage component V q ′ output from a torque component current controller  47   a  and torque component voltage command V q * output from a torque component voltage limiter  54   a,  a first integrator  2  for holding the torque component voltage saturation amount ΔV q , a magnetic flux command corrector  3   a  for outputting magnetic flux command correction amount Δφ 2d  from the held torque component voltage saturation amount ΔV q ′ and rotation angular speed ω to Cartesian two-axis coordinates, and a second subtracter  4  for subtracting the magnetic flux command correction amount Δφ 2d  from magnetic flux command φ 2d * and outputting magnetic flux correction command φ 2d * cmd .

TECHNICAL FIELD

This invention relates to a speed control apparatus of an AC motor andin particular to improvement in characteristic in a higher-speed areathan rated speed.

BACKGROUND OF THE INVENTION

In current control of an AC motor, often vector control is performedwherein the current of the AC motor is disassembled into an excitationcomponent (which will be hereinafter referred to as d axis) and a torquecomponent (which will be hereinafter referred to as q axis), ofcomponents on rotating Cartesian two-axis coordinates (which will behereinafter referred to as dq-axis coordinates) and the components arecontrolled separately. The case of an induction motor will be discussedbelow as a related art.

FIG. 16 is a drawing to show the configuration of a speed controlapparatus of an induction motor in a related art. In the figure, numeral31 denotes an induction motor, numeral 32 denotes a PWM inverter forsupplying electric power to the induction motor 31 based on voltagecommand Vu*, Vv*, Vw* described later, numerals 33 a, 33 b, and 33 cdenote current detectors for detecting currents i_(u), i_(v), and i_(w)of the induction motor 31, and numeral 34 denotes a speed detector fordetecting rotation speed ω_(r) of the induction motor 31. Numeral 35denotes a secondary magnetic flux calculator for calculating magneticflux φ_(2d) based on d-axis current i_(1d) described later, numeral 36denotes a slip frequency calculator for calculating slip angularfrequency ω_(s) based on q-axis current i_(1q) described later and themagnetic flux φ_(2d), numeral 37 denotes a coordinate rotation angularspeed calculator for calculating rotation angular speed ω of dq-axiscoordinates based on the slip angular frequency ω_(s) calculated by theslip frequency calculator 36 and the rotation speed ω_(r) of theinduction motor 31 detected by the speed detector 34, and numeral 38denotes an integrator for integrating the rotation angular speed ω andoutputting phase angle θ of dq-axis coordinates. Numeral 39 denotes athree-phase to two-phase coordinate converter for disassembling thecurrents i_(u), i_(v), and i_(w) of the current detectors 33 a, 33 b,and 33 c into the d-axis current i_(1d) and the q-axis current i_(1q) onthe dq-axis coordinates based on the phase angle θ of the dq-axiscoordinates and outputting the d-axis current i_(1d) and the q-axiscurrent i_(1q).

Numeral 40 denotes a subtracter for outputting magnetic flux deviatione_(f) between magnetic flux command φ_(2d)* and the magnetic flux φ_(2d)output by the secondary magnetic flux calculator 35, numeral 41 denotesa magnetic flux controller for controlling proportional integration(which will be hereinafter referred to as PI) so that the magnetic fluxdeviation e_(f) becomes 0 and outputting d-axis current componenti_(id)′, numeral 42 denotes a subtracter for outputting speed deviatione_(w) between speed commands ω_(r)* and the rotation speed ω_(r) of theinduction motor 31 output by the speed detector 34, and numeral 43denotes a speed controller for controlling PI so that the speeddeviation e_(w) becomes 0 and outputting q-axis current componenti_(1q)′.

Numeral 44 denotes a subtracter for outputting current deviation e_(1d)between d-axis current command i_(1d)* and d-axis current i_(1d),numeral 45 b denotes a d-axis current controller for controlling PI sothat the current deviation e_(1d) becomes 0 and outputting d-axisvoltage component V_(d)′, numeral 46 denotes a subtracter for outputtingcurrent deviation e_(1q) between q-axis current command i_(1q)* andq-axis current i_(1q), numeral 47 b denotes a q-axis current controllerfor controlling PI so that the current deviation e_(1q) becomes 0 andoutputting q-axis voltage component V_(q)′, and numeral 48 denotes atwo-phase to three-phase coordinate converter for converting d-axisvoltage command V_(d)* and q-axis voltage command V_(q)* into thevoltage commands Vu*, Vv*, and Vw* on the three-phase AC coordinatesbased on the phase angle θ of the dq-axis coordinates and outputting thevoltage commands as voltage commands of the PWM inverter 32.

Numeral 51 denotes a d-axis current limiter for limiting the d-axiscurrent component i_(1d)′ within a predetermined range and outputtingthe d-axis current command i_(1d)*, and numeral 52 denotes a q-axiscurrent limiter for limiting the q-axis current component i_(1q)′ withina predetermined range and outputting the q-axis current command i_(1q)*.Numeral 53 b denotes a d-axis voltage limiter for limiting the d-axisvoltage component V_(d)′ within a predetermined range and outputting thed-axis voltage command V_(d)*, and numeral 54 b denotes a q-axis voltagelimiter for limiting the q-axis voltage component V_(q)′ within apredetermined range and outputting the q-axis voltage command Vq*.

Numeral 55 denotes a magnetic flux command generation section forarbitrarily giving the magnetic flux command φ_(2d)* of the inductionmotor. The speed command ω_(r)* is given arbitrarily from the outside.

FIG. 17 is a drawing to show the configuration of the PI controller ofthe magnetic flux controller 41, the speed controller 43, the d-axiscurrent controller 45 b, the q-axis current controller 47 b, etc., inFIG. 16. In FIG. 17, numeral 61 denotes a coefficient unit correspondingto proportional gain K_(P) of the PI controller, numeral 62 denotes acoefficient unit corresponding to integration gain K_(I) of the PIcontroller, numeral 63 b denotes an integrator having a function ofstopping calculation, and numeral 64 denotes an adder for adding theproportional component and the integration component.

Letter e denotes deviation input to the PI controller and U′ denotescontrol input output from the PI controller. As for the magnetic fluxcontroller 41, e corresponds to the magnetic flux deviation e_(f)between the magnetic flux command φ_(d2)* and the magnetic flux φ_(2d)output by the secondary magnetic flux calculator 35, and U′ correspondsto the d-axis current component i_(1d)′. As for the speed controller 43,a corresponds to the speed deviation e_(w) between the speed commandω_(r)* and the rotation speed ω_(r) of the induction motor 31 output bythe speed detector 34, and U′ corresponds to the q-axis currentcomponent i_(1q)′. As for the d-axis current controller 45 b, ecorresponds to the current deviation e_(id) between the d-axis currentcommand i_(1d)* and the d-axis current i_(1d), and U′ corresponds to thed-axis voltage component V_(d)′. As for the q-axis current controller 47b, e corresponds to the current deviation e_(iq) between the q-axiscurrent command i_(1q)* and the q-axis current i_(1q), and U′corresponds to the q-axis voltage component V_(q)′.

The basic operation of the vector control in the induction motor will bediscussed with FIGS. 16 and 17.

As shown in FIG. 16, the vector control is implemented using a pluralityof PI controllers of the magnetic flux controller 41, the speedcontroller 43, the d-axis current controller 45 b, the q-axis currentcontroller 47 b, etc., in combination.

The subtracter at the stage preceding each PI controller (subtracter 40,subtracter 42, subtracter 44, subtracter 46) outputs deviation e (e_(f),e_(w), e_(id), e_(iq)) from the command value and actually detectedvalue.

The PI controller is a controller for setting the deviation output fromthe subtracter at the predetermined stage to 0 (matching the commandvalue and actually detected value with each other). Each PI controllerinputs the deviation e output from the subtracter at the preceding stageand outputs such control input U′ (i_(1d)′, i_(1q)′, V_(d)′, V_(q)′)setting the deviation e to 0 based on the following expression (1):

U′=(K _(P)+(K _(I) /s))₃ ·e   (1)

The block diagram of expression (1) is shown in FIG. 17, wherein K_(P)denotes the proportional gain of the PI controller and K_(I) denotes theintegration gain of the PI controller. The PI controller used in FIG. 16(magnetic flux controller 41, speed controller 43, d-axis currentcontroller 45 b, q-axis current controller 47 b) is the PI controllershown in FIG. 17, but the PI controllers differ in values of K_(P) andK_(I).

In the magnetic flux controller 41 or the speed controller 43, thed-axis current component i_(1d)′ or the q-axis current component i_(1q)′corresponds to the control input U′, but cannot be set to a value equalto or greater than maximum output current value i_(max) allowed by thePWM inverter 32. Then, the d-axis current limiter 51, the q-axis currentlimiter 52 limits so that the control input U′ output from the magneticflux controller 41, the speed controller 43 (d-axis current componenti_(1d)′, q-axis current component, i_(1q)′) does not exceed the maximumoutput current value i_(max) allowed by the PWM inverter 32.

In the d-axis current controller 45 b or the q-axis current controller47 b, the d-axis voltage component V_(d)′ or the q-axis voltagecomponent V_(q)′ corresponds to the control input U′, but cannot be setto a value equal to or greater than bus voltage V_(DC) of the PWMinverter 32. Thus, the d-axis voltage limiter 53 b, the q-axis voltagelimiter 54 b limits so that the control input U′ output from the d-axiscurrent controller 45 b or the q-axis current controller 47 b (d-axisvoltage component V_(d)′ or q-axis voltage component V_(q)′) does notexceed the bus voltage V_(DC) of the PWM inverter 32.

However, the limit values of the d-axis current limiter 51, the q-axiscurrent limiter 52, the d-axis voltage limiter 53 b, and the q-axisvoltage limiter 54 b need not necessarily be the same.

As described above, in the speed control apparatus of the inductionmotor in the related art, the limiters 51, 52, 53 b, and 54 b areprovided for outputs of the PI controllers 41, 43, 45 b, and 47 b and ifthe control input U′ is limited by the limiter 51, 52, 53 b, 54 b, inputdeviation e does not become 0 for ever and if the deviation e continuesto be accumulated in the integrator 63 b in the PI controller, aphenomenon called control input saturation arises, causing a vibratoryoutput response called overshoot or hunting; this is a problem.

Thus, if the control input U′ exceeds the limit value of the limiter 51,52, 53 b, 54 b, empirically the integration operation of the integrator63 b in the PI controller is stopped, thereby avoiding continuing toaccumulator the deviation e for eliminating control input saturation,thereby obtaining a stable response.

FIG. 18 is a graph plotting the d-axis voltage component V_(d)′ and theq-axis voltage component V_(q)′ based on expressions for findingterminal-to-terminal voltage in a stationary state in the inductionmotor described later: In the figure, (a), (c), and (e) indicate thed-axis voltage component V_(d)′ and (b), (d), and (f) indicate theq-axis voltage component V_(q)′.

FIG. 19 is a graph to show the limit values of the q-axis currentlimiter relative to the rotation speed ω_(r).

FIG. 20 is a graph to show the maximum allowable values of the magneticflux command φ_(2d)* that can be arbitrarily output from the magneticflux command generation section relative to the rotation speed ω_(r).

FIG. 18 corresponds to FIGS. 19 and 20. If the limit value is changed to(a), (c), and (e) in FIG. 19, the graph of FIG. 18 becomes as (a), (c),and (e). If the maximum allowable value is changed to (b), (d), and (f)in FIG. 20, the graph of FIG. 18 becomes as (b), (d), and (f).

To operate the induction motor at rated speed or more, the d-axisvoltage component V_(d)′ and the q-axis voltage component V_(q)′ outputfrom the d-axis current controller 45 b and the q-axis currentcontroller 47 b continue to exceed the limit values of the d-axisvoltage limiter 53 b and the q-axis voltage limiter 54 b stationarily.The above-described method of stopping the integration operation if thecontrol input exceeds the limit value is means for temporarily avoidingthe uncontrollable state of control input saturation and is effectivefor transient control input saturation, but cannot be used if controlinput saturation continues to occur stationarily as whether theinduction motor is operated at the rated speed or more.

A method in related art for eliminating control input saturation of thevoltage components V_(d)′ and V_(q)′ occurring stationarily at the ratespeed or more will be discussed with FIGS. 18 to 20. Such control inputsaturation of V_(d)′ and V_(q)′ in high-speed area is particularlycalled voltage saturation.

As for the induction motor, the d-axis voltage component V_(d)′ and theq-axis voltage component V_(q)′ in a stationary state are givenaccording to the following expressions (2) and (3):

V _(d) ′=R ₁ ·i _(1d) −L ₁ ·σ·ω·i _(1q)   (2)

V _(q) ′=R ₁ ·i _(1q)+(L ₁ /M)·ω·φ_(2d)   (3)

where R₁ denotes primary resistance of the induction motor 31, L₁denotes primary side self inductance, M denotes mutual inductance, and σdenotes a leakage coefficient.

To operate the induction motor at the rated speed or more, the secondterm component in expression (2), (3) becomes very larger than the firstterm component and thus expression (2) and (3) can be approximated bythe following expressions (4) and (5):

V _(d) ′=−L ₁ ·σ·ω·i _(1q)   (4)

V _(q)′=(L ₁ /M)·ω·φ_(2d)   (5)

The q-axis current limiter 52 is a fixed limiter and the q-axis currentlimiter value is indicated by FIG. 19 (a). Here, assuming that theq-axis current i_(1q) flows as much as the limit value, V_(d)′ becomesthe graph of FIG. 18 (a) according to expression (4). The maximumallowable value of φ_(2d)* that can be arbitrarily output from themagnetic flux command generation section 55 is indicated by FIG. 20 (b).Here, assuming that the magnetic flux φ_(2d) takes the same value as themaximum allowable value, V_(q)′ becomes the graph of FIG. 18 (b)according to expression (5).

From FIGS. 18 (a) and (b), it is seen that to operate the inductionmotor at rotation speed ω_(base) or more, the voltage component V_(q)′becomes saturated exceeding the output limit value of the PWM inverter32 ±V_(max) and that to operate the induction motor at rotation speedω_(base2) or more, both the voltage components V_(d)′ and V_(q)′ becomesaturated exceeding the output limit value of the PWM inverter 32±V_(max).

Since voltage saturation occurs stationarily in such an area at therated speed or more, the maximum allowable value of φ_(2d)* of themagnetic flux command generation section 55 and the limit value of theq-axis current limiter 52 are changed in response to the speed. Forexample, if a variable limiter is adopted for changing the limit valueof the q-axis current limiter in a manner inversely proportional to thespeed from the rotation speed ω_(base2) at which saturation of thed-axis component occurs as indicated by FIG. 19 (c), even if the q-axiscurrent i_(1q) flows as much as the limit value, V_(d)′ becomes thegraph of FIG. 18 (c) according to expression (4). If the maximumallowable value of φ_(2d)* that can be arbitrarily output from themagnetic flux command generation section 55 is limited by a functioninversely proportional to the speed from the rotation speed ω_(base) atwhich voltage saturation of the q-axis component occurs as indicated byFIG. 20 (d), even if the magnetic flux φ_(2d) takes the same value asthe maximum allowable value, V_(q)′ becomes the graph of FIG. 18 (d)according to expression (5).

As described above, the limit value of the q-axis current limiter andthe maximum allowable value of φ_(2d)* are changed in response to thespeed, whereby the d-axis voltage component V_(d)′ and the q-axisvoltage component V_(q)′ are prevented from exceeding the output limitvalue of the PWM inverter 32 ±V_(max) even in an area at the rated speedor more, and occurrence of voltage saturation can be suppressed, so thata stable response can be provided.

However, if the induction motor is actually turned, the voltagecomponent V_(d)′, V_(q)′ may become larger than FIG. 18 (c), (d) becauseof fluctuation of the magnitude of load or bus voltage, and voltagesaturation occurs, resulting in an unstable response.

Then, the limit value of the q-axis current limiter and the maximumallowable value of φ_(d2)* are set further lower as in FIG. 19 (e) andFIG. 20 (f) and the voltage component V_(d)′, V_(q)′ can be providedwith a margin relative to the output limit value of the RWM inverter 32±V_(max) as in FIGS. 18 (e), (f) for making voltage saturation hard tooccur.

In this case, however, it is made impossible to make full use of thecapabilities of the PWM inverter and lowering of output torque or thelike is incurred; this is a problem.

To make voltage saturation hard to occur without lowering the outputtorque, a method of feeding back a magnetic flux command or a currentcommand for correction if voltage saturation occurs is proposed. In thismethod, when voltage saturation occurs, the saturation amount isdetected, an optimum correction amount to eliminate the voltagesaturation is formed from the saturation amount, and each command iscorrected. Such feedback control is performed, whereby occurrence ofvoltage saturation can be suppressed and stability of control can beimproved independently of the conditions of the load and the busvoltage, and it is also made possible to make full use of thecapabilities of the PWM inverter.

For example, the Unexamined Japanese Patent Application No. 2000-92899discloses a control apparatus of an induction motor, comprising avoltage saturation compensation circuit for making a comparison betweena voltage command value from a current control system and a bus voltagevalue of a PWM inverter and integrating and if the above-mentioned busvoltage value is greater than the above-mentioned voltage command value,the voltage saturation compensation circuit for subtracting theabove-mentioned integrated output from a magnetic flux command and ifthe above-mentioned bus voltage value is lower than the above-mentionedvoltage command value, for subtracting 0 from the magnetic flux command.

In this method, the correction amount is derived in response to thevoltage saturation amount and each command is corrected, so that voltagesaturation can be eliminated; however, since the speed of the motor isnot considered when the correction amount is determined, to cope withrapid speed change, etc., calculation of the correction amount, etc.,must be thought out in such a manner that the correction amount isincreased to make a prompt correction at the acceleration time and thatthe correction amount is suppressed to raise stability at thedeceleration time, for example; the method involves such a problem.

The invention is intended for solving the problems as described aboveand it is an object of the invention to provide a speed controlapparatus of an AC motor for making it possible to suppress occurrenceof voltage saturation without performing special operation even if rapidspeed change, etc., occurs.

DISCLOSURE OF THE INVENTION

According to the invention, there is provided a speed control apparatusof an AC motor having current controllers for performing proportionalintegration control of an excitation component current and a torquecomponent current of two components on rotating Cartesian two-axiscoordinates into which a current of the AC motor is separated, the speedcontrol apparatus comprising:

a torque component voltage limiter for limiting a torque componentvoltage component output from torque component current controller forperforming proportional integration control of the torque componentcurrent so that the torque component voltage component becomes equal toor less than a predetermined value; a first subtracter for finding atorque component voltage saturation amount from the torque componentvoltage component output from the above-mentioned torque componentcurrent controller and a torque component voltage command output fromthe above-mentioned torque component voltage limiter; a first integratorfor holding the torque component voltage saturation amount; a magneticflux command corrector for outputting a magnetic flux command correctionamount from the held torque component voltage saturation amount androtation angular speed of Cartesian two-axis coordinates; and a secondsubtracter for subtracting the magnetic flux command correction amountfrom a magnetic flux command and outputting a magnetic flux correctioncommand, so that if the speed rapidly changes, etc., the optimumcorrection amount can always be obtained and occurrence of voltagesaturation can be suppressed.

Also, there is provided a speed control apparatus of an AC motor havingcurrent controllers for performing proportional integration control ofan excitation component current and a torque component current of twocomponents on rotating Cartesian two-axis coordinates into which acurrent of the AC motor is separated, the speed control apparatuscomprising:

a torque component voltage limiter for limiting a torque componentvoltage component output from torque component current controller forperforming proportional integration control of the torque componentcurrent so that the torque component voltage component becomes equal toor less than a predetermined value; a first subtracter for finding atorque component voltage saturation amount from the torque componentvoltage component output from the above-mentioned torque componentcurrent controller and a torque component voltage command output fromthe above-mentioned torque component voltage limiter; a first integratorfor holding the torque component voltage saturation amount; anexcitation component current command corrector for outputting anexcitation component current command correction amount from the heldtorque component voltage saturation amount and rotation angular speed ofCartesian two-axis coordinates; and a third subtracter for subtractingthe excitation component current command correction amount from anexcitation component current command and outputting an excitationcomponent current command correction command, so that

if the speed rapidly changes, etc., the optimum correction amount canalways be obtained and it is possible to suppress occurrence of voltagesaturation.

Rotation speed of the above-mentioned AC motor is input to a magneticflux command generation section for generating a magnetic flux commandand a magnetic flux command is generated in response to the rotationspeed of the above-mentioned AC motor, so that the magnitude of thetorque component voltage saturation amount can be lessened to someextent and it is made possible to improve the stability of control ofthe AC motor.

Rotation speed of the above-mentioned AC motor is input to an excitationcomponent current command generation section for generating anexcitation component current command and an excitation component currentcommand is generated in response to the rotation speed of theabove-mentioned AC motor, so that the magnitude of the torque componentvoltage saturation amount can be lessened to some extent and it is madepossible to improve the stability of control of the AC motor.

The speed control apparatus comprises an excitation component voltagelimiter for limiting an excitation component voltage component outputfrom excitation component current controller for performing proportionalintegration control of the excitation component current so that theexcitation component voltage component becomes equal to or less than apredetermined value; a fourth subtracter for finding the excitationcomponent voltage component output from the above-mentioned excitationcomponent current controller and an excitation component voltagesaturation amount output from the above-mentioned excitation componentvoltage limiter; a second integrator for holding the excitationcomponent voltage saturation amount; an excitation component currentcommand corrector for outputting a torque component current commandcorrection amount from the held excitation component voltage saturationamount and rotation angular speed of Cartesian two-axis coordinates; anda fifth subtracter for subtracting the torque component current commandcorrection amount from a torque component current command and outputtinga torque component current correction command, so that to operate the ACmotor in an area wherein the speed largely exceeds the rated speed,occurrence of voltage saturation can also be suppressed and it ispossible to perform stable control.

In a torque component current limiter for limiting a torque componentcurrent command output from a speed controller for performingproportional integration control of speed deviation between a speedcommand and the rotation speed of the AC motor so that the torquecomponent current command becomes equal to or less than a predeterminedvalue, the limit value for limiting the torque component current commandis varied in response to the rotation speed of the above-mentioned ACmotor, so that the magnitude of the extinction component voltagesaturation amount can be lessened to some extent and it is made possibleto improve the stability of control of the AC motor.

According to the invention, there is provided a speed control apparatusof an AC motor having current controllers for performing proportionalintegration control of an excitation component current and a torquecomponent current of two components on rotating Cartesian two-axiscoordinates into which a current of the AC motor is separated, whereintorque component current controller for performing proportionalintegration control of the torque component current is configured so asto continue calculation of an internal integrator even if torquecomponent voltage component becomes saturated, the speed controlapparatus comprising a torque component voltage limiter for limiting thetorque component voltage component output from the torque componentcurrent controller for performing proportional integration control ofthe torque component current so that the torque component voltagecomponent becomes equal to or less than a predetermined value; a firstsubtracter for finding a torque component voltage saturation amount fromthe torque component voltage component output from the above-mentionedtorque component current controller and a torque component voltagecommand output from the above-mentioned torque component voltagelimiter; a magnetic flux command corrector for outputting a magneticflux command correction amount from the torque component voltagesaturation amount and rotation angular speed of Cartesian two-axiscoordinates; and a second subtracter for subtracting the magnetic fluxcommand correction amount from a magnetic flux command and outputting amagnetic flux correction command, so that

occurrence of voltage saturation can be suppressed according to thesample configuration.

Also, there is provided a speed control apparatus of an AC motor havingcurrent controllers for performing proportional integration control ofan excitation component current and a torque component current of twocomponents on rotating Cartesian two-axis coordinates into which acurrent of the AC motor is separated, wherein

torque component current controller for performing proportionalintegration control of the torque component current is configured so asto continue calculation of an internal integrator even if torquecomponent voltage component becomes saturated, the speed controlapparatus comprising a torque component voltage limiter for limiting atorque component voltage component output from torque component currentcontroller for performing proportional integration control of the torquecomponent current so that the torque component voltage component becomesequal to or less than a predetermined value; a first subtracter forfinding a torque component voltage saturation amount from the torquecomponent voltage component output from the above-mentioned torquecomponent current controller and a torque component voltage commandoutput from the above-mentioned torque component voltage limiter; anexcitation component current command corrector for outputting anexcitation component current command correction amount from the torquecomponent voltage saturation amount and rotation angular speed ofCartesian two-axis coordinates; and a third subtracter for subtractingthe excitation component current command correction amount from anexcitation component current command and outputting an excitationcomponent current command correction command, so that

occurrence of voltage saturation can be suppressed according to thesimple configuration.

Rotation speed of the above-mentioned AC motor is input to a magneticflux command generation section for generating a magnetic flux commandand a magnetic flux command is generated in response to the rotationspeed of the above-mentioned AC motor, so that the magnitude of thetorque component voltage saturation amount can be lessened to someextent and it is made possible to improve the stability of control ofthe AC motor according to the simple configuration.

Rotation speed of the above-mentioned AC motor is input to an excitationcomponent current command generation section for generating anexcitation component current command and an excitation component currentcommand is generated in response to the rotation speed of theabove-mentioned AC motor, so that the magnitude of the torque componentvoltage saturation amount can be lessened to some extent and it is madepossible to improve the stability of control of the AC motor accordingto the simple configuration.

Excitation component current controller for performing proportionalintegration control of the excitation component current is configured soas to continue calculation of an internal integrator even if excitationcomponent voltage component becomes saturated, and the speed controlapparatus comprises an excitation component voltage limiter for limitingthe excitation component voltage component output from the excitationcomponent current controller for performing proportional integrationcontrol of the excitation component current so that the excitationcomponent voltage component becomes equal to or less than apredetermined value; a fourth subtracter for finding the excitationcomponent voltage component output from the above-mentioned excitationcomponent current controller and an excitation component voltagesaturation amount output from the above-mentioned excitation componentvoltage limiter; an excitation component current command corrector foroutputting a torque component current command correction amount from theexcitation component voltage saturation amount and rotation angularspeed of Cartesian two-axis coordinates; and a fifth subtracter forsubtracting the torque component current command correction amount froma torque component current command and outputting a torque componentcurrent correction command, so that

to operate the AC motor in an area wherein the speed largely exceeds therated speed, occurrence of voltage saturation can also be suppressed andit is possible to perform stable control according to the simpleconfiguration.

In a torque component current limiter for limiting a torque componentcurrent command output from a speed controller for performingproportional integration control of speed deviation between a speedcommand and the rotation speed of the AC motor so that the torquecomponent current command becomes equal to or less than a predeterminedvalue, the limit value for limiting the torque component current commandis varied in response to the rotation speed of the above-mentioned ACmotor, so that the magnitude of the excitation component voltagesaturation amount can be lessened to some extent and it is made possibleto improve the stability of control of the AC motor according to thesimple configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing to show the configuration of a speed controlapparatus of an induction motor according to a first embodiment of theinvention.

FIG. 2 is a graph plotting d-axis voltage component V_(d)′ and q-axisvoltage component V_(q)′ based on expressions (4) and (5) for findingterminal-to-terminal voltage in a stationary state in the inductionmotor described above.

FIG. 3 is a drawing to show the configuration of a magnetic flux commandcorrector 3 a, 3 b in the speed control apparatus of the induction motoraccording to the first embodiment of the invention.

FIG. 4 is a drawing to show the configuration of a speed controlapparatus of a permanent-magnet motor according to a second embodimentof the invention.

FIG. 5 is a drawing to show the configuration of a d-axis currentcommand corrector 5 a, 5 b according to the second embodiment of theinvention.

FIG. 6 is a drawing to show the configuration of a speed controlapparatus of an induction motor according to a third embodiment of theinvention.

FIG. 7 is a drawing to show the configuration of a speed controlapparatus of a permanent-magnet motor according to the third embodimentof the invention.

FIG. 8 is a drawing to show the configuration of a speed controlapparatus of an induction motor according to a fourth embodiment of theinvention.

FIG. 9 is a graph plotting d-axis voltage component V_(d)′ and theq-axis voltage component V_(q)′ based on expressions (4) and (5) forfinding terminal-to-terminal voltage in a stationary state in theinduction motor.

FIG. 10 is a drawing to show to the configuration of a q-axis currentcommand corrector 13 a, 13 b according to the fourth embodiment of theinvention.

FIG. 11 is a drawing to show the configuration of a speed controlapparatus of a permanent-magnet motor according to the fourth embodimentof the invention.

FIG. 12 is a drawing to show the configuration of a speed controlapparatus of an induction motor according to a fifth embodiment of theinvention.

FIG. 13 is a drawing to show the configuration of a speed controlapparatus of an induction motor according to a sixth embodiment of theinvention.

FIG. 14 is a drawing to show the configuration of a PI controller of ad-axis current controller 16, a q-axis current controller 17, etc., inFIG. 13.

FIG. 15 is a drawing to show the configuration of a speed controlapparatus of a permanent-magnet motor according to the sixth embodimentof the invention.

FIG. 16 is a drawing to show the configuration of a speed controlapparatus of an inductor motor in a related art.

FIG. 17 is a drawing to show the configuration of a PI controller of amagnetic flux controller 41, a speed controller 43, a d-axis currentcontroller 45 b, a q-axis current controller 47 b, etc., in FIG. 16.

FIG. 18 is a graph plotting d-axis voltage component V_(d)′ and q-axisvoltage component V_(q)′ based on expressions for findingterminal-to-terminal voltage in a stationary state in the inductionmotor described later.

FIG. 19 is a graph to show the limit values of the q-axis currentlimiter relative to rotation speed ω_(r).

FIG. 20 is a graph to show the maximum allowable values of magnetic fluxcommand φ_(2d)* that can be arbitrarily output from a magnetic fluxcommand generation section relative to the rotation speed ω_(r).

BEST MODE FOR CARRYING OUT THE INVENTION

First Embodiment

FIG. 1 is a drawing to show the configuration of a speed controlapparatus of an induction motor according to a first embodiment of theinvention. In the figure, numerals 31 to 39, 40 to 44; 45 b, 46, 48, 51,52, 53 b, and 55 are similar to those in FIG. 16 and will not bediscussed again. Numeral 1 denotes a first subtracter for outputtingq-axis voltage saturation amount ΔV_(q) from q-axis voltage componentV_(q) ¹ and q-axis voltage command V_(q)*, numeral 2 denotes a firstintegrator for holding the q-axis voltage saturation amount ΔV_(q) ¹,numeral 3 a denotes a magnetic flux command corrector for outputtingmagnetic flux command correction amount ΔΦ_(2d) from the held q-axisvoltage saturation amount ΔV_(q) ¹, and rotation angular speed ω ofdq-axis coordinates, and numeral 4 denotes a second subtracter foroutputting magnetic flux correction command Φ_(2d)*_(cmd) resulting fromsubtracting the magnetic flux command correction amount ΔΦ_(2d) frommagnetic flux command Φ_(2d)*.

Numeral 47 a denotes a q-axis current controller for controlling PI sothat current deviation e_(iq) becomes 0 and outputting the q-axisvoltage component V_(q)′, and numeral 54 a denotes a q-axis voltagelimiter for limiting the q-axis voltage component V_(q)′ within apredetermined range and outputting the q-axis voltage command V_(q)*.

FIG. 2 is a graph plotting d-axis voltage component V_(d)′ and theq-axis voltage component V_(q)′ based on expressions (4) and (5) forfinding terminal-to-terminal voltage in a stationary state in theinduction motor described above; (a) indicates a graph of d-axis voltagecommand V_(d)* before being corrected according to the first embodiment,(b) indicates a graph of q-axis voltage command V_(q)* before beingcorrected according to the first embodiment, and (c) indicates a graphof q-axis voltage command V_(q)* after being corrected according to thefirst embodiment.

FIG. 3 is a drawing to show the configuration of the magnetic fluxcommand corrector 3 a in the speed control apparatus of the inductionmotor according to the first embodiment of the invention. In the figure,numeral 21 denotes a divider for dividing the held q-axis voltagesaturation amount ΔV_(q)′ by the rotation angular speed ω of dq-axiscoordinates, and numeral 22 denotes a coefficient unit for inputtingoutput of the divider 21 and outputting the magnetic flux commandcorrection amount Δφ_(2d). However, in a magnetic flux command corrector3 b described later, divider 21 divides the q-axis voltage saturationamount ΔV_(q) by the rotation angular speed ω of dq-axis coordinates.

The operation of the speed control apparatus of the induction motoraccording to the first embodiment will be discussed with FIGS. 1 to 3,FIG. 19, and FIG. 20. When voltage saturation does not occur, the speedcontrol apparatus operates in a similar manner to that in the relatedart and the operation of the speed control apparatus will not bediscussed again.

The terminal-to-terminal voltage of the induction motor in a stationarystate is given according to expressions (4) and (5), as described abovein the related art example.

A q-axis current limiter 52 is a fixed limiter with a limit valueindicated by FIG. 19 (a). Assuming that a q-axis current i_(1q) flows asmuch as the limit value, V_(d)′ becomes the graph of FIG. 2 (a)according to expression (4). The maximum allowable value of φ_(2d)* thatcan be arbitrarily output from a magnetic flux command generationsection 55 is indicated by FIG. 20 (b). Assuming that magnetic flux 100_(2d) takes the same value as the maximum allowable value, V_(q)′becomes the graph of FIG. 2 (b) according to expression (5).

To operate the induction motor in an area wherein the speed is abouttwice the rated speed (rotation speed ω_(base)), the d-axis voltagecomponent V_(d)′ does not exceed output limit value ±V_(max) asindicated by FIG. 2 (a). However, to operate the induction motor in anarea wherein the speed is equal to or higher than the rotation speedω_(base), the q-axis voltage component V_(q)′ exceeds output limit value±V_(max) and voltage saturation occurs. If voltage saturation occurs,the q-axis voltage component V_(q)′ is limited to ±V_(max) by the q-axisvoltage limiter 54 a. The input/output value of the q-axis voltagelimiter 54 a is passed through the subtracter 1, whereby deviation(which will be hereinafter referred to as q-axis voltage saturationamount ΔV_(q)) can be found. The q-axis voltage saturation amount ΔV_(q)is a parameter indicating how much voltage is saturated, and correspondsto the V_(q)′ difference indicated by FIGS. 2 (b) and (c).

In expression (5), L₁ and M are parameters of induction motor and arefixed and the speed ω needs to be made as commanded because of the speedcontrol apparatus and cannot be corrected. Thus, it is seen that whenvoltage saturation occurs due to the q-axis voltage component V_(q)′,the magnetic flux φ_(2d) must be corrected to a lower value to suppressV_(q)′. That is, the correction amount Δφ_(2d) to the magnetic flux isfound from the held q-axis voltage saturation amount ΔV_(q)′ and themagnetic flux is corrected to a lower value based on the correctionamount, whereby voltage saturation is eliminated.

The relationship between the held q-axis voltage saturation amountΔV_(q)′ and the magnetic flux command correction amount Δφ_(2d) toeliminate voltage saturation is represented by expression (6) similar toexpression (5).

ΔV _(q)′=(L ₁ /M)·ω·Δφ_(2d)   (6)

Further, if expression (6) is deformed with respect to the magnetic fluxcommand correction amount Δφ_(2d), it results in expression (7).

Δφ_(2d)=(M/L ₁)·ΔV _(q)′/ω  (7)

Expression (7) becomes an expression for finding the correction amountΔφ_(2d) to the magnetic flux from the held q-axis voltage saturationamount ΔV_(d)′ and corresponds to the magnetic flux command corrector 3a in FIG. 1 and a specific block diagram thereof is shown in FIG. 3.

The magnetic flux command correction amount Δφ_(2d) obtained asmentioned above is input to the subtracter 4 and the magnetic fluxcommand φ_(2d)* is corrected to a lower value of the magnetic fluxcorrection command φ_(2d)*_(cmd). According to the correction, the graphof the q-axis voltage component V_(q)′ plotted based on the expressionof the terminal-to-terminal voltage becomes FIG. 2 (c) and occurrence ofvoltage saturation of the q-axis component can be suppressed.

In the first embodiment, if q-axis voltage saturation occurs, the degreeof the voltage saturation is detected as the q-axis voltage saturationamount, the optimum magnetic flux command correction amount foreliminating the voltage saturation is determined in response to theq-axis voltage saturation amount, and the magnetic flux command iscorrected in a feedback manner.

When the correction amount is determined, the speed of the motor isconsidered. Thus, if the speed changes rapidly, etc., the optimumcorrection amount can always be obtained and it is possible to suppressoccurrence of voltage saturation.

Stable control can be performed independently of change in theconditions of the load and the bus voltage, and the capabilities of thePWM inverter can always be exploited at the maximum, so that it is madepossible to increase output torque, etc.

The example of the induction motor has been described as the AC motor,but similar means can be used not only for the induction motor, but alsofor a synchronous motor for which magnetic flux control can beperformed, needless to say.

Second Embodiment

FIG. 4 is a drawing to show the configuration of a speed controlapparatus of a permanent-magnet motor according to a second embodimentof the invention. In the figure, numerals 1, 2, 32 to 34, 38, 39, 42 to44, 45 b, 47 a, 48, 52, 53 b, and 54 a are similar to those in FIG. 1and will not be discussed again.

Numeral 5 a denotes a d-axis current command corrector for inputtingheld q-axis voltage saturation amount ΔV_(q)′ and rotation angular speedω of dq-axis coordinates and outputting d-axis current commandcorrection amount Δi_(1d), and numeral 6 denotes a third subtracter foroutputting d-axis current correction command i_(1d)*_(cmd) corrected bysubtracting the d-axis current command correction amount Δi_(1d) fromd-axis current command i_(1d)*. Numeral 56 denotes a permanent-magnetmotor, numeral 57 denotes a d-axis current command generation sectionfor outputting an arbitrary d-axis current command, and numeral 58denotes a coefficient unit for calculating coordinate rotation angularspeed.

In the first embodiment, the example of the speed control apparatus forcontrolling the induction motor has been shown; the second embodimentrelates to the speed control apparatus for controlling apermanent-magnet motor as an AC motor.

In FIG. 4, as compared with FIG. 1 showing the configuration of thespeed control apparatus for controlling the induction motor, as the ACmotor to be controlled, the induction motor 31 is replaced with apermanent-magnet motor 56, the magnetic flux command generation section55, the subtracter 4, the magnetic flux command corrector 3 a, thesecondary magnetic flux calculator 35, the slip frequency calculator 36,the coordinate rotation angular speed calculator 37, the subtracter 40,the magnetic flux controller 41, and the current limiter 51 are deleted,and the d-axis current command generation section 57 for outputting anarbitrary d-axis current command, the coefficient unit 58 forcalculating coordinate rotation angular speed, and the subtracter 6 arenewly added. The speed control apparatus for controlling thepermanent-magnet motor differs from the speed control apparatus forcontrolling the induction motor slightly in basic configuration, butthey perform the same basic operation and also involve the same problemto be solved.

FIG. 5 is a drawing to show the configuration of the d-axis currentcommand corrector 5 a according to the second embodiment of theinvention. In the figure, numeral 23 denotes a divider for dividing theheld q-axis voltage saturation amount ΔV_(q)′ by the rotation angularspeed ω of dq-axis coordinates, and numeral 24 denotes a coefficientunit for inputting output of the divider 23 and outputting the d-axiscurrent command correction amount Δi_(1d). However, in a d-axis currentcommand corrector 5 b described later, divider 23 divides the q-axisvoltage saturation amount ΔV_(q) by the rotation angular speed ω ofdq-axis coordinates.

As for the permanent-magnet motor, the d-axis voltage component V_(d)′and the q-axis voltage component V_(q)′ in a stationary state are givenaccording to the following expressions (8) and (9):

V _(d) ′=R ₁ ·i _(1d) −L _(q) ·ω·i _(1q)   (8)

V _(q) ′=R _(i) ·i _(1q)+ω(L _(d) ·i _(1d)+φ_(f))   (9)

where R₁ denotes primary resistance of the permanent-magnet motor 56,L_(d) denotes d-axis component inductance, L_(q)* denotes q-axiscomponent inductance, and φ_(f) denotes the maximum value of fluxlinkage produced by the permanent magnet.

To operate the permanent-magnet motor at the rated speed or more, eachsecond term component becomes very larger than the first time componentand thus expression (8) can be approximated by expression (10) andexpression (9) can be approximated by expression (11):

V _(d) ′=−L _(q) ·ω·i _(1q)   (10)

V _(q)′=ω(L _(d) ·i _(1d)+φ_(f))   (11)

The operation of the speed control apparatus according to the secondembodiment will be discussed with FIGS. 4 and 5. When voltage saturationdoes not occur, the speed control apparatus operates in a similar mannerto that in the related art and the operation of the speed controlapparatus will not be discussed again.

In the first embodiment, when voltage saturation occurs due to theq-axis voltage component V_(q)′, a correction is made to the magneticflux command to eliminate the voltage saturation; in the secondembodiment, the AC motor comprising no magnetic flux control system, thepermanent-magnet motor, is to be controlled and thus the correctionmethod is as follows:

If voltage saturation occurs, the q-axis voltage component V_(q)′ islimited to ±V_(max) by a q-axis voltage limiter 54 a. The input/outputvalue of the q-axis voltage limiter 54 a is passed through a subtracter1, whereby deviation (which will be hereinafter referred to as q-axisvoltage saturation amount ΔV_(q)) can be found. The q-axis voltagesaturation amount ΔV_(q) is a parameter indicating how much voltage issaturated.

In expression (11), L_(d) and φ_(f) are parameters of permanent-magnetmotor and are fixed and the speed ω needs to be made as commandedbecause of the speed control apparatus and cannot be corrected. Thus, itis seen that when voltage saturation occurs due to the q-axis voltagecomponent V_(q)′, d-axis current i_(1d) must be corrected to a lowervalue to suppress V_(q)′. That is, the correction amount Δi_(1d) to thed-axis current is found from the held q-axis voltage saturation amountΔV_(q)′ and the d-axis current is corrected to a lower value based onthe correction amount, whereby voltage saturation is eliminated.

The relationship between the held q-axis voltage saturation amountΔV_(q)′ and the d-axis current command correction amount Δi_(1d) toeliminate voltage saturation can be thought according to expression(12).

ΔV _(q) ′=ω·L _(d) ·Δi _(1d)   (12)

If expression (12) is deformed with respect to the d-axis currentcommand correction amount Δi_(1d), expression (13) is obtained.

Δi _(1d) ′=ΔV _(q)′/(ω·L _(d))   (13)

Expression (13) becomes an expression for deriving the correction amountΔi_(1d) to the d-axis current from the held q-axis voltage saturationamount ΔV_(q)′ and corresponds to the d-axis current command corrector 5a in FIG. 4 and a specific block diagram thereof is shown in FIG. 5.

The d-axis current command correction amount Δi_(1d) thus obtained isinput to the subtracter 6 and d-axis current command i_(id)* iscorrected to a lower value of the d-axis current correction commandi_(1d)*_(cmd). According to the correction, occurrence of voltagesaturation of the q-axis component can be suppressed.

As described above, according to the second embodiment, in the AC motorcomprising no magnetic flux control system, occurrence of voltagesaturation can also be suppressed if the speed rapidly changes as in thefirst embodiment, stable control can be performed independently ofchange in the conditions of load and bus voltage, and the capabilitiesof the PWM inverter can always be exploited at the maximum, so that itis made possible to increase output torque, etc.

Similar means can be used not only for the permanent-magnet motor, butalso for the induction motor comprising no magnetic flux control system,needless to say. The permanent-magnet motors include an SPM motor havingno silent-pole property wherein L_(d)=L_(q) and an IPM motor havingsilent-pole property wherein L_(d)<L_(q), but in the invention, thetechnique can be applied to any permanent-magnet motors regardless ofthe presence or absence of silent-pole property.

Third Embodiment

FIG. 6 is a drawing to show the configuration of a speed controlapparatus of an induction motor according to a fourth embodiment of theinvention. In the figure, numerals 1, 2, 3 a, 4, 31 to 39, 40 to 45, 45b, 46, 47 a, 48, 51, 52, 53 b, and 54 a similar to those in FIG. 1 andwill not be discussed again. Numeral 7 denotes a magnetic flux commandgeneration section for inputting rotation speed ω_(r) of an inductionmotor 31 and outputting magnetic flux command φ_(2d)* of the inductionmotor in response to the rotation speed ω_(r).

The operation of the speed control apparatus of an AC motor according tothe third embodiment will be discussed with FIGS. 6 and 2.

In the first embodiment, the example is shown wherein if q-axis voltagesaturation occurs, the magnetic flux command correction amount Δφ_(2d)found from the held q-axis voltage saturation amount ΔV_(q)′ and therotation angular speed ω of the dq-axis coordinates is subtracted fromthe magnetic flux command φ_(2d)* output from the magnetic flux commandgeneration section 55 to generate the magnetic flux correction commandφ_(2d)*_(cmd). To suppress occurrence of voltage saturation; themagnetic flux correction command φ_(2d)*_(cmd) may be decreased inresponse to an increase in the rotation speed ω_(r).

The magnetic flux command generation section 55 generally outputs aconstant value (magnetic flux command φ_(2d)*). Thus, when the rotationspeed ω_(r) increases, unless the magnetic flux command correctionamount Δφ_(2d) is increased, it becomes impossible to suppressoccurrence of voltage saturation. As shown in FIG. 2, as the rotationspeed ω_(r) increases, the q-axis voltage saturation amount ΔV_(q)grows. However, to perform stable control, it is not much preferred thatthe fed-back correction amount becomes too large.

In the third embodiment, the rotation speed ω_(r) is input to themagnetic flux command generation section 9 and the magnetic flux commandφ_(2d)* is varied in response to the rotation speed ω_(r). For example,the magnetic flux command φ_(2d)* is varied in such a manner that themagnetic flux command φ_(2d)* is weakened in inverse proportion to anincrease in the rotation speed ω_(r).

The magnetic flux command φ_(2d)* output from the magnetic flux commandgeneration section 9 is changed in response to an increase in therotation speed ω_(r), whereby the q-axis voltage saturation amountΔV_(q) can be lessened and the magnetic flux command correction amountΔφ_(2d) fed back as the correction amount can be suppressed.

As described above, according to the third embodiment, the rotationspeed ω_(r) is input to the magnetic flux command generation section 7and the magnetic flux command φ_(2d)* to be output is variedaccordingly, so that the magnitude of the magnetic flux commandcorrection amount Δφ_(2d) fed back can be lessened to some extent and itis made possible to improve the stability of control of the AC motor.

FIG. 7 is a drawing to show the configuration of a speed controlapparatus of a permanent-magnet motor according to the third embodimentof the invention. In FIG. 6, the example is shown wherein the magneticflux command generation section 55 for outputting a constant value(magnetic flux command φ_(2d)*) in the speed control apparatus of theinduction motor in the first embodiment is replaced with the magneticflux command generation section 7 for varying the magnetic flux commandφ_(2d)* in response to the rotation speed ω_(r). In FIG. 7, the d-axiscurrent command generation section 57 for outputting an arbitrary d-axiscurrent command i_(1d)* in the second embodiment is replaced with ad-axis current command generation section 8 for varying d-axis currentcommand i_(1d)* in response to the rotation speed ω_(r).

In control of the permanent-magnet motor, the magnitude of d-axiscurrent command correction amount Δi_(1d) fed back can also be lessenedto some extent and it is made possible to improve the stability as withthe induction motor.

Fourth Embodiment

FIG. 8 is a drawing to show the configuration of a speed controlapparatus of an induction motor according to a fourth embodiment of theinvention. In the figure, numerals 1, 2, 3 a, 4, 31 to 39, 40 to 44, 46,47 a, 48, 51, 52, 54 a, and 55 are similar to those in FIG. 1 and willnot be discussed again.

Numeral 11 denotes a fourth subtracter for outputting d-axis voltagesaturation amount ΔV_(d) from d-axis voltage component V_(d)′ and d-axisvoltage command V_(d)*, numeral 12 denotes an integrator for holding thed-axis voltage saturation amount ΔV_(d) and outputting held d-axisvoltage saturation amount ΔV_(d)′, numeral 13 a denotes a q-axis currentcommand corrector for inputting the held d-axis voltage saturationamount ΔV_(d)′ and rotation angular speed ω of dq-axis coordinates andoutputting q-axis current command correction amount Δi_(1q), and numeral14 denotes a fifth subtracter for outputting q-axis current correctioncommand i_(1q)*_(cmd) corrected by subtracting the q-axis currentcommand correction amount Δi_(1q) from q-axis current command i_(1q)*.Numeral 45 a denotes a d-axis current controller for controlling PI sothat current deviation e_(id) becomes 0 and outputting the d-axisvoltage component V_(d)′, and numeral 53 a denotes a d-axis voltagelimiter for limiting the d-axis voltage component V_(d)′ within apredetermined range and outputting the d-axis voltage command V_(d)*.

FIG. 9 is a graph plotting d-axis voltage component V_(d)′ and theq-axis voltage component V_(q)′ based on expressions (4) and (5) forfinding terminal-to-terminal voltage in a stationary state in theinduction motor; (a) indicates a graph of d-axis voltage componentV_(d)′ before being corrected according to the fourth embodiment, (b)indicates a graph of q-axis voltage component V_(q)′ before beingcorrected according to the fourth embodiment, (d) indicates a graph ofq-axis voltage component V_(q)′ after being corrected according to thefourth embodiment, and (d) indicates a graph of d-axis voltage componentV_(d)′ after being corrected according to the fourth embodiment.

FIG. 10 is a drawing to show the configuration of the q-axis currentcommand corrector 13 a according to the fourth embodiment of theinvention. In the figure, numeral 25 denotes a divider for dividing theheld d-axis voltage saturation amount ΔV_(d)′ by the rotation angularspeed ω of dq-axis coordinates, and numeral 26 denotes a coefficientunit for inputting output of the divider 25 and outputting the q-axiscurrent command correction amount Δi_(1q). However, in a magnetic fluxcommand corrector 13 b described later, divider 25 divides the d-axisvoltage saturation amount ΔV_(d) by the rotation angular speed ω ofdq-axis coordinates.

In the first embodiment to the third embodiment, the example wherein themotor is operated in an area wherein the speed is about twice the ratedspeed (rotation speed ω_(base)) is shown. The fourth embodiment makes itpossible to cope with the case where a motor is operated in an areawherein the speed largely exceeds the rated speed.

The operation of the speed control apparatus of the induction motoraccording to the fourth embodiment will be discussed with FIGS. 8 to 10,FIG. 19, and FIG. 20. When voltage saturation does not occur, the speedcontrol apparatus operates in a similar manner to that in the relatedart and the operation of the speed control apparatus will not bediscussed again.

The terminal-to-terminal voltage of the induction motor in a stationarystate is given according to expressions (4) and (5), as described in therelated art. A q-axis current limiter 52 is a fixed limiter and itsq-axis current limit value is indicated by FIG. 19 (a). Assuming that aq-axis current i_(1q) flows as much as the limit value, V_(d)′ becomesthe graph of FIG. 9 (a) according to expression (4). The maximumallowable value of φ_(2d)* that can be arbitrarily output from amagnetic flux command generation section 55 is indicated by FIG. 20 (b).Assuming that magnetic flux φ_(2d) takes the same value as the maximumallowable value, V_(q)′ becomes the graph of FIG. 9 (b) according toexpression (5).

As shown in FIGS. 9 (a) and (b), to operate the induction motor in anarea wherein the speed largely exceeds the rated speed, the q-axisvoltage component V_(q)′ exceeds output limit value ±V_(max) and voltagesaturation occurs in an area wherein the speed is equal to or higherthan the rotation speed ω_(base), and further the d-axis voltagecomponent V_(d)′ also exceeds output limit value ±V_(max) and voltagesaturation occurs in a high-speed area wherein the speed is equal to orhigher than rotation speed ω_(base2). Here, if voltage saturation of theq-axis voltage component V_(q)′ occurs in an area wherein the speed isequal to or higher than the rotation speed ω_(base) (however, less thanthe rotation speed ω_(base2)), the speed control apparatus operates in asimilar manner to that of the speed control apparatus of the AC motorshown above in each of the first embodiment to the third embodiment, andthe operation of the speed control apparatus will not be discussedagain. The q-axis voltage component V_(q)′ plotted based on theexpression of the terminal-to-terminal voltage is indicated by graph ofFIG. 8 (c) and occurrence of voltage saturation of the q-axis componentcan be suppressed.

Further, if voltage saturation of the d-axis voltage command V_(d)*occurs in an area wherein the speed is equal to or higher than therotation speed ω_(base2), the d-axis voltage component V_(d)′ is limitedto ±V_(max) by the d-axis voltage limiter 53 a. The input/output valueof the d-axis voltage limiter 53 a is passed through a subtracter 1,whereby deviation (which will be hereinafter referred to as d-axisvoltage saturation amount ΔV_(d)) can be found. The d-axis voltagesaturation amount ΔV_(d) is a parameter indicating how much voltage issaturated, and corresponds to the V_(d)′ difference indicated by FIGS. 9(d) and (a).

According to expression (4), L₁ and σ are parameters of induction motorand are fixed ad the speed ω needs to be made as commanded because ofthe speed control apparatus and cannot be corrected. Thus, it is seenthat when voltage saturation occurs due to the d-axis voltage componentV_(d)′, the q-axis current i_(1q) must be corrected to a lower value tosuppress V_(d)′. That is, the correction amount Δi_(1q) to the q-axiscurrent is found from the held d-axis voltage saturation amount ΔV_(d)′and the q-axis current is corrected to a lower value based on thecorrection amount, whereby voltage saturation is eliminated.

The relationship between the d-axis voltage saturation amount ΔV_(d)′and the q-axis current command correction amount Δi_(1q) to eliminatevoltage saturation is given according to expression (14) as inexpression (4).

ΔV _(d) ′=L ₁ ·σ·ω·Δi _(1q)   (14)

If expression (14) is deformed with respect to the q-axis currentcommand correction amount Δi_(1q); it results in expression (15).

Δi _(1q) =−ΔV _(d)′/−(L ₁·σ·ω)   (15)

Expression (15) becomes an expression for deriving the correction amountΔi_(1q) to the q-axis current from the held d-axis voltage saturationamount ΔV_(d)′ and corresponds to the q-axis current command corrector13 a in FIG. 8 and a specific block diagram thereof is shown in FIG. 10.

The q-axis current command correction amount Δi_(1q) thus obtained isinput to the subtracter 8 and the q-axis current command i_(1q)* iscorrected to a lower value of the q-axis current correction commandi_(1q)*_(cmd). According to the correction, the graph of the d-axisvoltage component V_(d)′ plotted based on the theoretical expression ofthe terminal-to-terminal voltage becomes FIG. 9 (d) and occurrence ofvoltage saturation of the d-axis component can be suppressed.

As described above, according to the fourth embodiment, if d-axisvoltage saturation occurs, the degree of the voltage saturation isdetected as the d-axis voltage saturation amount, and the optimum q-axiscurrent command correction amount for eliminating the voltage saturationis determined in response to the d-axis voltage saturation amount and isfed back to correct the q-axis current command.

When the correction amount is determined, the speed of the motor isconsidered. Thus, if the speed changes rapidly, etc., the optimumcorrection amount can always be obtained and it is possible to suppressoccurrences of voltage saturation.

To operate the AC motor in an area wherein the speed largely exceeds therated speed, stable control can also be performed independently ofchange in the conditions of load and bus voltage, and the capabilitiesof the PWM inverter can always be exploited at the maximum, so that itis made possible to increase output torque, etc.

FIG. 11 is a drawing to show the configuration of a speed controlapparatus of a permanent-magnet motor according to the fourth embodimentof the invention. In the figure, numerals 1, 1, 5 a, 6, 32 to 34, 38,39, 42 to 44, 46, 47 a, 48, 54 a, and 56 to 58 are similar to those inFIG. 4 shown in the second embodiment and will not be discussed again.Numeral 11 denotes a fourth subtracter for outputting d-axis voltagesaturation amount ΔV_(d) from d-axis voltage component V_(d)′ and d-axisvoltage command V_(d)*, numeral 12 denotes an integrator for holding thed-axis voltage saturation amount ΔV_(d) and outputting held d-axisvoltage saturation amount ΔV_(d)′, numeral 13 a denotes a q-axis currentcommand corrector for inputting the held d-axis voltage saturationamount ΔV_(d)′ and rotation angular speed ω of dq-axis coordinates andoutputting q-axis current command correction amount Δi_(1q), and numeral14 denotes a fifth subtracter for outputting q-axis current correctioncommand i_(1q)*_(cmd) corrected by subtracting the q-axis currentcommand correction amount Δi_(1q) from q-axis current command i_(1q)*.Numeral 45 a denotes a d-axis current controller for controlling PI sothat current deviation e_(id) becomes 0 and outputting the d-axisvoltage component V_(d)′, and numeral 53 a denotes a d-axis voltagelimiter for limiting the d-axis voltage component V_(d)′ within apredetermined range and outputting the d-axis voltage command V_(d)*.

FIG. 11 shows an example of using the fourth embodiment of the inventionfor the speed control apparatus of the permanent-magnet motor, and theoperation of the speed control apparatus is similar to that of the speedcontrol apparatus of the induction motor in FIG. 8 and will not bediscussed again.

Fifth Embodiment

FIG. 12 is a drawing to show the configuration of a speed controlapparatus of an induction motor according to a fifth embodiment of theinvention. In the figure, numerals 1, 2, 3 a, 4, 11, 12, 13 a, 14, 31 to39, 40 to 44, 45 a, 46, 47 a, 48, 51, 53 a, 54 a, and 55 are similar tothose in FIG. 8 and will not be discussed again. Numeral 15 denotes aq-axis current limiter for inputting rotation speeds ω_(r) of aninduction motor 31 and varying a limit value in response to the rotationspeed ω_(r).

The operation of the speed control apparatus of the AC motor accordingto the fifth embodiment will be discussed with FIGS. 12 and 9.

In the fourth embodiment, if d-axis voltage saturation occurs, thedegree of the voltage saturation is detected as the d-axis voltagesaturation amount ΔV_(d), and the optimum q-axis current commandcorrection amount Δi_(1q) for eliminating the voltage saturation isdetermined in response to the d-axis voltage saturation amount and theq-axis current command correction amount Δi_(1q) is fed back to correctthe q-axis current command I_(1q)*. Here, as shown in FIG. 9 for thefourth embodiment, as the rotation speed ω_(r) increases, the d-axisvoltage saturation amount ΔV_(d) grows. However, to perform stablecontrol, it is not much preferred that the fed-back correction amountbecomes too large.

In the fourth embodiment, the result of subtracting the q-axis currentcommand correction amount Δi_(1q) fed back as the correction amount fromthe q-axis current command i_(1q)* output from the q-axis currentlimiter 52 becomes the final q-axis current correction commandi_(1q)*_(cmd). To suppress occurrence of voltage saturation, the q-axiscurrent correction command i_(1q)*_(cmd) may be small for an increase inthe rotation speed ω_(r).

However, the q-axis current limiter 52 in the fourth embodiment is afixed limiter and always limits with a constant value. Thus, when therotation speed ω_(r) increases and the q-axis current command is outputfully up to the limit value, unless the q-axis current commandcorrection amount Δi_(1q) is increased, it becomes impossible tosuppress occurrence of voltage saturation.

In the fifth embodiment, the q-axis current limiter 52 of a fixedlimiter in the fourth embodiment is replaced with the q-axis currentlimiter 15 of a variable limiter for varying the limit value in responseto the rotation speed ω_(r). For example, the limit value is varied insuch a manner that the limit value is weakened in inverse proportion toan increase in the rotation speed ω_(r).

The q-axis current command i_(1q)* output from the q-axis currentlimiter 15 is variably limited for an increase in the rotation speedω_(r), whereby the d-axis voltage saturation amount ΔV_(d) can belessened, and the q-axis current command correction amount Δi_(1q)feedback as the correction amount can be suppressed.

The control example of the induction motor has been shown. However, inthe speed control apparatus of the permanent-magnet motor in FIG. 11,the q-axis current limiter 52 of a fixed limiter is replaced with theq-axis current limiter 15 of a variable limiter for varying the limitvalue in response to the rotation speed ω_(r), whereby stability canalso be improved in control of the permanent-magnet motor.

As described above, according to the fifth embodiment, the q-axiscurrent limiter 15 is made a variable limiter for varying the limitvalue in response to the rotation speed ω_(r), so that the magnitude ofthe d-axis voltage saturation amount ΔV_(d) can be lessened to someextent and it is made possible to improve the stability of control ofthe AC motor.

It is also possible to use the fifth embodiment and the third embodimentin combination, lessen both the d-axis voltage saturation amount ΔV_(d)and the q-axis voltage saturation amount ΔV_(q) to some extent, andremarkably improve the stability of control of the AC motor.

Sixth Embodiment

FIG. 13 is a drawing to show the configuration of a speed controlapparatus of an induction motor according to a sixth embodiment of theinvention. In the figure, numerals 1, 4, 11, 14, 31 to 39, 40 to 44, 46,48, 51, 52, and 55 are similar to those in FIG. 8 and will not bediscussed again. Numeral 3 b denotes a magnetic flux command correctorfor outputting magnetic flux command correction amount Δφ_(2d) fromq-axis voltage saturation amount ΔV_(q) and rotation angular speed ω ofdq-axis coordinates, numeral 13 b denotes a q-axis current commandcorrector for inputting d-axis voltage saturation amount ΔV_(d) androtation angular speed ω of dq-axis coordinates and outputting q-axiscurrent command correction amount Δi_(1q), numeral 16 denotes a d-axiscurrent controller for controlling PI so that current deviation e_(id)becomes 0 and outputting d-axis voltage component V_(d)′, numeral 17denotes a q-axis current controller for controlling PI so that currentdeviation e_(iq) becomes 0 and outputting q-axis voltage componentV_(q)′, numeral 18 denotes a d-axis voltage limiter for limiting thed-axis voltage component V_(d)′ within a predetermined range andoutputting d-axis voltage command V_(d)*, and numeral 19 denotes aq-axis voltage limiter for limiting the q-axis voltage component V_(q)′within a predetermined range and outputting q-axis voltage commandV_(q)*.

FIG. 14 is a drawing to show the configuration of a PI controller of acurrent controller 16, 17 used in the speed control apparatus of theinduction motor according to the sixth embodiment of the invention. Inthe figure, numerals 61, 62, and 64 are similar to those in FIG. 17 ofthe related art example and will not be discussed again. Numeral 63 adenotes an integrator.

Letter e denotes deviation input to the PI controller and U′ denotescontrol input output from the PI controller. As for the d-axis currentcontroller 16, e corresponds to the current deviation e_(id) betweend-axis current command i_(1d)* and d-axis current i_(1d), and U′corresponds to the d-axis voltage component V_(d)′. As for the q-axiscurrent controller 17, e corresponds to current deviation e_(iq) betweenthe q-axis current command i_(1q)* and q-axis current i_(1q), and U′corresponds to the q-axis voltage component V_(q)′.

If the controller input U′ exceeds the limit value of the d-axis voltagelimiter 53 a, 53 b, the q-axis voltage limiter 54 a, 54 b, the d-axiscurrent controller 45 a, 45 b, the q-axis current controller 47 a, 47 bused in the related art example and the first embodiment to the fifthembodiment is configured for stopping the calculation of the integrator63 in the current controller for controlling PI and thus the integrator12 for holding the d-axis voltage saturation amount ΔV_(d) and theintegrator 2 for holding the q-axis voltage saturation amount ΔV_(q) areadded. However, even if the control input U′ exceeds the limit value ofthe d-axis voltage limiter 18, the q-axis voltage limiter 19, the d-axiscurrent controller 16 and the q-axis current controller 17 used in thesixth embodiment cause each a value equal to or greater than the limitvalue to be held in the internal integrator 63 a without stopping thecalculation of the integrator 63 a in the current controller forcontrolling PI.

In the sixth embodiment, the d-axis current controller 45 a and theq-axis current controller 47 a in the fourth embodiment are replacedwith the d-axis current controller 16 and the q-axis current controller17, the integrator 12 for holding the d-axis voltage saturation amountΔV_(d) and the integrator 2 for holding the q-axis voltage saturationamount ΔV_(q) are eliminated, the magnetic flux command corrector 3 afor outputting the magnetic flux command correction amount Δφ_(2d) fromthe q-axis voltage saturation amount ΔV_(q)′ held in the integrator 2and the rotation angular speed ω of dq-axis coordinates is replaced withthe magnetic flux command corrector 3 b for outputting the magnetic fluxcommand correction amount Δφ_(2d) from the q-axis voltage saturationamount ΔV_(q) and the rotation angular speed ω of dq-axis coordinates,and the q-axis current command corrector 13 a for inputting the d-axisvoltage saturation amount ΔV_(d)′ held in the integrator 12 and therotation angular speed ω of dq-axis coordinates and outputting theq-axis current command correction amount Δi_(1q) is replaced with theq-axis current command corrector 13 b for inputting the d-axis voltagesaturation amount ΔV_(d) and the rotation angular speeds ω of dq-axiscoordinates and outputting the q-axis current command correction amountΔi_(1q), whereby equal operation is performed. The operation of thespeed control apparatus is similar to that of the speed controlapparatus of the fourth embodiment and therefore will not be discussedagain.

The example wherein the current controllers 45 a and 47 a in FIG. 8 arereplaced with the current controllers 16 and 17 has been described, butthe current controllers 45 a and 47 a in FIG. 12 may be replaced withthe current controllers 16 and 17. The current controller 47 a in FIG. 6may be replaced with the current controller 17.

The d-axis current controller 16 and the q-axis current controller 17 ofthe PI controllers designed for not stopping the calculation of theintegrators 63 a in the PI controller even if the control input U′exceeds the limit value are used, so that occurrence of voltagesaturation can be suppressed according to the simple configuration.

FIG. 15 is a drawing to show the configuration of a speed controlapparatus of a permanent-magnet motor according to the sixth embodimentof the invention. In the figure, numerals 1, 11, 32 to 34, 38, 39, 42 to44, 46, 48, 51, 52, and 56 to 58 are similar to those in FIG. 11 andwill not be discussed again. Numeral 5 b denotes a d-axis currentcommand corrector for inputting q-axis voltage saturation amount ΔV_(q)and rotation angular speed ω of dq-axis coordinates and outputtingd-axis current command correction amount Δi_(1d), numeral 13 b denotes aq-axis current command corrector for inputting d-axis voltage saturationamount ΔV_(d) and rotation angular speed ω of dq-axis coordinates andoutputting q-axis current command correction amount Δi_(1q), numeral 16denotes a d-axis current controller for controlling PI so that currentdeviation e_(id) becomes 0 and outputting d-axis voltage componentV_(d)′, numeral 17 denotes a q-axis current controller for controllingPI so that current deviation e_(iq) becomes 0 and outputting q-axisvoltage component V_(q)′, numeral 18 denotes a d-axis voltage limiterfor limiting the d-axis voltage component V_(d)′ within a predeterminedrange and outputting d-axis voltage command V_(d)*, and numeral 19denotes a q-axis voltage limiter for limiting the q-axis voltagecomponent V_(q)′ within a predetermined range and outputting q-axisvoltage command V_(q)*.

In FIG. 15, the d-axis current controller 45 a and the q-axis currentcontroller 47 a in FIG. 11 are replaced with the d-axis currentcontroller 16 and the q-axis current controller 17 each for causing avalue equal to or greater than the limit value to be held in theinternal integrator 63 a without stopping the calculation of theintegrator 63 a in the current controller for controlling PI even if thecontrol input U′ exceeds the limit value of the d-axis voltage limiter18, the q-axis voltage limiter 19, the integrator 12 for holding thed-axis voltage saturation amount ΔV_(d) and the integrator 2 for holdingthe q-axis voltage saturation amount ΔV_(q) are eliminated, the magneticflux command corrector 3 a for outputting the magnetic flux commandcorrection amount Δφ_(2d) from the q-axis voltage saturation amountΔV_(q)′ held in the integrator 2 and the rotation angular speed ω of thedq-axis coordinates is replaced with the magnetic flux command corrector3 b for outputting the magnetic flux command correction amount Δφ_(2d)from the q-axis voltage saturation amount ΔV_(q) and the rotationangular speed ω of the dq-axis coordinates, and the q-axis currentcommand corrector 13 a for inputting the d-axis voltage saturationamount ΔV_(d)′ held in the integrator 12 and the rotation angular speedω of dq-axis coordinates and outputting the q-axis current commandcorrection amount Δi_(1q) is replaced with the q-axis current commandcorrector 13 b for inputting the d-axis voltage saturation amount ΔV_(d)and the rotation angular speed ω of dq-axis coordinates and outputtingthe q-axis current command correction amount Δi_(1q), whereby equaloperation is performed. The operation of the speed control apparatus issimilar to that of the speed control apparatus of the fourth embodimentand therefore will not be discussed again.

The example wherein the current controllers 45 a and 47 a in FIG. 11 arereplaced with the current controllers 16 and 17 has been described, butthe current controller 47 a in FIG. 4 may be replaced with the currentcontroller 17.

The d-axis current controller 16 and the q-axis current controller 17 ofthe PI controllers designed for not stopping the calculation of theintegrator 63 a in the PI controller even if the control input U′exceeds the limit value are used, so that occurrence of voltagesaturation can be suppressed according to the simple configuration.

INDUSTRIAL APPLICABILITY

As described above, if voltage saturation occurs in the speed controlapparatus of the AC motor, the optimum correction amount for eliminatingthe voltage saturation is found based on the voltage saturation amountdetected as the voltage saturation degree and is fed back to correcteach command, so that the speed control apparatus is suited for use inapplication wherein high-speed operation at the rated speed or higher isperformed or rapid speed change is made.

What is claimed is:
 1. A speed control apparatus of an AC motor havingcurrent controllers for performing proportional integration control ofan excitation component current and a torque compound current of twocomponents on rotating Cartesian two-axis coordinates into which acurrent of the AC motor is separated, said speed control apparatuscomprising: a torque component voltage limiter for limiting a torquecomponent voltage component output from torque component currentcontroller for performing proportional integration control of the torquecomponent current so that the torque component voltage component becomesequal to or less than a predetermined value; a first subtracter forfinding a torque component voltage saturation amount from the torquecomponent voltage component output from said torque component currentcontroller and a torque component voltage command output from saidtorque component voltage limiter; a first integrator for holding thetorque component voltage saturation amount; a magnetic flux commandcorrector for outputting a magnetic fluid command correction amount fromthe held torque component voltage saturation amount and rotation angularspeed of Cartesian two-axis coordinates; and a second subtracter forsubtracting the magnetic flux command correction amount from a magneticflux command and outputting a magnetic flux correction command.
 2. Thespeed control apparatus of an AC motor as claimed in claim 1, whereinrotation speed of said AC motor is input to a magnetic flux commandgeneration section for generating a magnetic flux command, and amagnetic flux command is generated in response to the rotation speed ofsaid AC motor.
 3. The speed control apparatus of an AC motor as claimedin claim 1 comprising: an excitation component voltage limiter forlimiting an excitation component voltage component output fromexcitation component current controller for performing proportionalintegration control of the excitation component current so that theexcitation component voltage component becomes equal to or less than apredetermined value; a fourth subtracter for finding the excitationcomponent voltage component output from said excitation componentcurrent controller and an excitation component voltage saturation amountoutput from said excitation component voltage limiter; a secondintegrator for holding the excitation component voltage saturationamount; an excitation component current command corrector for outputtinga torque component current command correction amount from the heldexcitation component voltage saturation amount and rotation angularspeed of Cartesian two-axis coordinates; and a fifth subtracter forsubtracting the torque component current command correction amount froma torque component current command and outputting a torque componentcurrent correction command.
 4. The speed control apparatus of an ACmotor as claimed in claim 3, wherein in a torque component currentlimiter for limiting a torque component current command output from aspeed controller for performing proportional integration control ofspeed deviation between a speed command and the rotation speed of saidAC motor so that the torque component current command becomes equal toor less than a predetermined value, the limit value for limiting thetorque component current command is varied in response to the rotationspeed of said AC motor.
 5. A speed control apparatus of an AC motorhaving current controllers for performing proportional integrationcontrol of an excitation component current and a torque componentcurrent of two components on rotating Cartesian two-axis coordinatesinto which a current of the AC motor is separated, said speed controlapparatus comprising: a torque component voltage limiter for limiting atorque component voltage component output from torque component currentcontroller for performing proportional integration control of the torquecomponent current so that the torque component voltage component becomesequal to or less than a predetermined value; a first subtracter forfinding a torque component voltage saturation amount from the torquecomponent voltage component output from said torque component currentcontroller and a torque component voltage command output from saidtorque component voltage limiter; a first integrator for holding thetorque component voltage saturation amount; an excitation componentcurrent command corrector for outputting an excitation component currentcommand correction amount from the held torque component voltagesaturation amount and rotation angular speed of Cartesian two-axiscoordinates; and a third subtracter for subtracting the excitationcomponent current command correction amount from an excitation componentcurrent command and outputting an excitation component current commandcorrection command.
 6. The speed control apparatus of an AC motor asclaimed in claim 2, wherein rotation speed of said AC motor is input toan excitation component current command generation section forgenerating an excitation component current command, and an excitationcomponent current command is generated in response to the rotation speedof said AC motor.
 7. The speed control apparatus of an AC motor asclaimed in claim 5 comprising: an excitation component voltage limiterfor limiting an excitation component voltage component output fromexcitation component current controller for performing proportionalintegration control of the excitation component current so that theexcitation component voltage component becomes equal to or less than apredetermined value; a fourth subtracter for finding the excitationcomponent voltage component output from said excitation componentcurrent controller and an excitation component voltage saturation amountoutput from said excitation component voltage limiter; a secondintegrator for holding the excitation component voltage saturationamount; an excitation component current command corrector for outputtinga torque component current command correction amount from the heldexcitation component voltage saturation amount and rotation angularspeed of Cartesian two-axis coordinates; and a fifth subtracter forsubtracting the torque component current command correction amount froma torque component current command and outputting a torque componentcurrent correction command.
 8. The speed control apparatus of an ACmotor as claimed in claim 7, wherein in a torque component currentlimiter for limiting a torque component current command output from aspeed controller for performing proportional integration control ofspeed deviation between a speed command and the rotation speed of saidAC motor so that the torque component current command becomes equal toor less than a predetermined value, the limit value for limiting thetorque component current command is varied in response to the rotationspeed of said AC motor.
 9. A speed control apparatus of an AC motorhaving current controllers for performing proportional integrationcontrol of an excitation component current and a torque componentcurrent of two components on rotating Cartesian two-axis coordinatesinto which a current of said AC motor is separated; said speed controlapparatus comprising: a torque component current controller forperforming proportional integration control of the torque componentcurrent is configured so as to continue calculation of an internalintegrator even if torque component voltage component becomes saturated;a torque component voltage limiter for limiting the torque componentvoltage component output from said torque component current controllerfor performing proportional integration control of the torque componentcurrent so that the torque component voltage component becomes equal toor less than a predetermined value; a first subtracter for finding atorque component voltage saturation amount from the torque componentvoltage component output from said torque component current controllerand a torque component voltage command output from said torque componentvoltage limiter; a magnetic flux command corrector for outputting amagnetic flux command correction amount from the torque componentvoltage subtraction amount and rotation angular speed of Cartesiantwo-axis coordinates; and a second subtracter for subtracting themagnetic flux command correction amount from a magnetic flux command andoutputting a magnetic flux correction command.
 10. The speed controlapparatus of an AC motor as claimed in claim 9, wherein rotation speedof said AC motor is input to a magnetic flux command generation sectionfor generating a magnetic flux command, and a magnetic flux command isgenerated in response to the rotation speed of said AC motor.
 11. Thespeed control apparatus of an AC motor as claimed in claim 9, said speedcontrol apparatus comprising: an excitation component current controllerfor performing proportional integration control of the excitationcomponent is configured so as to continue calculation of an internalintegrator even if excitation component voltage component becomessaturated; an excitation compound voltage limiter for limiting theexcitation compound voltage component output from said excitationcomponent current controller for performing proportional integrationcontrol of the excitation component current so that the excitationcomponent voltage component becomes equal to or less than apredetermined value; a fourth subtracter for finding the excitationcomponent voltage component output from said excitation componentcurrent controller and an excitation component voltage saturation amountoutput from said excitation component voltage limiter; an excitationcomponent current command corrector for outputting a torque componentcurrent command correction amount from the excitation component voltagesaturation amount and rotation angular speed of Cartesian two-axiscoordinates; and a fifth subtracter for subtracting the torque componentcurrent command correction amount from a torque component currentcommand and outputting a torque component current correction command.12. The speed control apparatus of an AC motor as claimed in claim 11,wherein in a torque component current limiter for limiting a torquecomponent current command output from a speed controller for performingproportional integration control of speed deviation between a speedcommand and the rotation speed of said AC motor so that the torquecomponent current command becomes equal to or less than a predeterminedvalue, the limit value for limiting the torque component current commandis varied in response to the rotation speed of said AC motor.
 13. Aspeed control apparatus of an AC motor having current controllers forperforming proportional integrating control of an excitation componentcurrent and a torque component current of two components on rotatingCartesian two-axis coordinates into which a current of said AC motor isseparated, said speed control apparatus comprising: a torque componentcurrent controller for performing proportional integration control ofthe torque component current is configured so as to continue calculationof an internal integrator even if the torque component voltage componentbecomes saturated; a torque component value limiter for limiting atorque component voltage component output from torque component currentcontroller for performing proportional integration control of the torquecomponent current so that the torque component voltage component becomesequal to or less than a predetermined value; a first subtracter forfinding a torque component voltage saturation amount from the torquecomponent voltage component output from said torque component currentcontroller and a torque component voltage command output from saidtorque component voltage limiter; an excitation component currentcommand corrector for outputting an excitation component current commandcorrection amount from the torque component voltage saturation amountand rotation angular speed of Cartesian two-axis coordinates; and athird subtracter for subtracting the excitation component currentcommand correction amount from an excitation component current commandand outputting an excitation component current command correctioncommand.
 14. The speed control apparatus of an AC motor as claimed inclaim 13, wherein rotation speed of said AC motor is input to anexcitation component current command generation section for generatingan excitation component current command, and an excitation componentcurrent command is generated in response to the rotation speed of saidAC motor.
 15. The speed control apparatus of an AC motor as claimed inclaim 13, said speed control apparatus comprising: an excitationcomponent current controller for performing proportional integrationcontrol of the excitation component current is configured so as tocontinue calculation of an internal integrator even if excitationcomponent voltage component becomes saturated; an excitation componentvoltage limiter for limiting an excitation component voltage componentoutput from excitation component current controller for performingproportional integration control of the excitation component current sothat the excitation component voltage component becomes equal to or lessthan a predetermined value; a fourth subtracter for finding theexcitation component voltage component output from said excitationcomponent current controller and an excitation component voltagesaturation amount output from said excitation component voltage limiter;an excitation component current command corrector for outputting atorque component current command correction amount from the excitationcomponent voltage saturation amount and rotation angular speed ofCartesian two-axis coordinates; and a fifth subtracter for subtractingthe torque component current command correction amount from a torquecomponent current command and outputting a torque component currentcorrection command.
 16. The speed control apparatus of an AC motor asclaimed in claim 15, wherein in a torque component current limiter forlimiting a torque component current command output from a speedcontroller for performing proportional integration control of speeddeviation between a speed command and the rotation speed of said ACmotor so that the torque component current command becomes equal to orless than a predetermined value, the limit value for limiting the torquecomponent current command is varied in response to the rotation speed ofsaid AC motor.